Well, that's not strictly true. The one thing to combat it is the same thing that you'd use to combat any network security threat - keep yourself patched up, don't run any network-aware apps you don't need, and ensure you've got a half-decent firewall in place.

Rootkits require a remote-root exploit to get onto your box; the only difference between this and a virus is that you need prevention, rather than just relying on your AV software to cure an infection after it's started.